1. Field of the Invention
This invention relates to a catalyst for the production of acrylic acid and a method for the production of acrylic acid by the use of this catalyst. More particularly, it relates to a catalyst for producing acrylic acid by the vapor-phase catalytic oxidation of acrolein or an acrolein-containing gas and to a method for the production of acrylic acid by the vapor-phase catalytic oxidation of acrolein or an acrolein-containing gas in the presence of this catalyst.
2. Description of the Prior Art
Various improved catalysts have been proposed for the efficient production of acrylic acid by the vapor-phase catalytic oxidation reaction of acrolein or an acrolein-containing gas. Most of the catalysts which have been heretofore proposed have molybdenum and vanadium as main components thereof. For example, JP-B-44-12,129 discloses a catalyst comprising molybdenum, vanadium and tungsten, JP-B-49-11,371 a catalyst comprising molybdenum, vanadium, copper, tungsten and chromium, JP-B-50-25,914 a catalyst comprising molybdenum and vanadium, JP-B-52-85,091 a catalyst comprising molybdenum, vanadium, copper and (at least one element selected among antimony and germanium).
These conventional molybdenum-vanadium type catalysts, however, are deficient not only in the yield of acrylic acid in the light of the industrial production but also in the service life of the catalyst because of the rapid decrease of the activity. The development of a catalyst for the production of acrylic acid which excels in activity and stability, and further permits the production of acrylic acid at high yield and stably for a long time has been longed for.
Solid acids whose magnitudes of acid strength (Ho) (hereinafter referred to simply as xe2x80x9cacid strengthxe2x80x9d or occasionally as xe2x80x9cHoxe2x80x9d) are not more than xe2x88x9211.93 are generally called as solid super acids as introduced in detail in xe2x80x9cSHOKUBAI,xe2x80x9d Vol. 31, No. 7 (1989), pp. 512 through 518, for example. According to the literature, the super acid is defined as an acid possessing acidity stronger than that of 100% sulfuric acid (Hoxe2x89xa6xe2x88x9211.93) and is reported to be usable under more moderate conditions than the ordinary acid catalysts in such reactions as decomposition, isomerization, alkylation, polymerization, acylation, dehydration, and dehydrogenation of hydrocarbon which are referred to as acid catalytic reactions. The fact that this super acid, particularly when combined with a molybdenum-vanadium type catalyst, is effective in inducing the vapor-phase catalytic oxidation reaction of acrolein or acrolein-containing gas has not been known at all to the art.
One object of this invention, therefore, is to provide a catalyst for producing acrylic acid at a high yield by the vapor-phase catalytic oxidation reaction of acrolein or acrolein-containing gas.
Another object of this invention is to provide a catalyst for the production of acrylic acid, which permits the production of acrylic acid by the vapor-phase catalytic oxidation reaction of acrolein or acrolein-containing gas stably for a long period.
Still another object of this invention is to provide a catalyst for the production of acrylic acid by the vapor-phase catalytic oxidation reaction of acrolein or acrolein-containing gas, which catalyst excels in terms of catalytic activity and service life and permits stable production of acrylic acid at high productivity for a long time.
Yet another object of this invention is to provide a method for producing acrylic acid efficiently by the use of the catalyst for the production of acrylic acid mentioned above.
The objects mentioned above are accomplished by a catalyst for producing acrylic acid by the oxidation of acrolein or acrolein-containing gas in a vapor phase with a molecular oxygen-containing gas, which catalyst comprises (A) a composite oxide having molybdenum and vanadium as essential components and adapted for the production of acrylic acid by the vapor-phase catalytic oxidation of acrolein and (B) a solid acid having acid strength (Ho) of not more than xe2x88x9211.93 (Hoxe2x89xa6xe2x88x9211.93). These objects are further accomplished, in the vapor-phase catalytic oxidation reaction for producing acrylic acid by the oxidation of acrolein or acrolein-containing gas in the vapor phase with the molecular oxygen-containing gas, by a method for the production of acrylic acid which effects the reaction in the presence of the catalyst for the production of acrylic acid mentioned above.
The present inventors have learnt that a catalyst composition which combines a molybdenum-vanadium type composite oxide which has been usually known as a catalyst for the production of acrylic acid with a solid acid having acid strength of not more than xe2x88x9211.93 possesses high activity at low temperatures as compared to conventional catalysts and excels in stability as a catalyst. This invention have been perfected on the basis of this knowledge.
Since the catalyst of this invention maintains high activity, it aids in the production of acrylic acid at a high yield.
Since the catalyst of this invention excels in terms of service life and maintains this outstanding quality for a long time, it permits acrylic acid to be produced stably for a long time. Even after a protracted use, this catalyst can continue the reaction for the production of acrylic acid at the same degree of yield as during the initial stage of the reaction without appreciably increasing the reaction temperature.
Since the catalyst of this invention exhibits high activity even at low temperatures, it aids in effecting the reaction at the same degree of yield at reaction temperatures lower than those necessary for the conventional methods.
Since the catalyst of this invention suffers no degradation of catalytic property even under high-load operation conditions aimed at a high productivity, it permits acrylic acid to be produced stably with high operational efficiency for a long time.
By the method of this invention, acrylic acid can be produced efficiently and advantageously on a commercial scale.
Now, this invention will be described in detail below.
Component (A):
As the component (A), any of the well-known composite oxide catalysts which have molybdenum and vanadium as essential components and are used for producing acrylic acid by the vapor-phase catalytic oxidation of acrolein can be used. In these catalysts, those catalysts which are represented by the following general formula (1):
MoaVbWcCudXeOg
(wherein Mo is molybdenum, V is vanadium, W is tungsten, Cu is copper, X is at least one element selected from the group consisting of Mg, Ca, Sr and Ba, and 0 is oxygen, and a, b, c, d, e, and g are atomic ratios respectively of Mo, V, W, Cu, X, and 0 such that b is a numeral in the range of 2 to 14, c in the range of 0 to 12, d in the range of 0 to 6 excluding 0 (0 less than dxe2x89xa66) (0.1 to 6, for example), e in the range of 0 to 3, and g is a numeral to be determined by the oxidized states of the elements when a is fixed at 12), are advantageously used. Particularly, the catalysts which are represented by the following general formula (2):
MoaVbWcCudXeyfOg
(wherein Mo is molybdenum, V is vanadium, W is tungsten, Cu is copper, X is at least one element selected from the group consisting of Mg, Ca, Sr and Ba, Y is at least one element selected from the group consisting of Ti, Zr, Ce, Cr. Mn, Fe, Co, Ni, Zn, Nb, Sn, Sb, Pb and Bi, and 0 is oxygen, and a, b, c, d, e, f, and g are atomic ratios respectively of Mo, V, W, Cu. X, Y, and 0 such that b is a numeral in the range of 2 to 14, c in the range of 0 to 12, d in the range of 0 to 6 excluding 0 (0 less than dxe2x89xa66) (0.1 to 6, for example), e in the range of 0 to 3, f in the range of 0 to 3, and g is a numeral to be determined by the oxidized states of the elements when a is fixed at 12), are advantageously used.
These catalysts are not discriminated on account of their methods of preparation. They may be prepared by any of the well-known methods. The compound containing the relevant elemental component as a starting material is not particularly limited by its kind. Any of the oxides which contain the relevant elemental component or the compounds which produce the oxides when calcined can be used. As typical examples of the compound which produces the oxides when calcined, hydroxides, metallic acids, nitrates, carbonates, ammonium salts, acetates, and formates may be cited. The compound which contains two or more elemental components cited above is likewise usable.
Generally, the composite oxide of the component (A) is obtained by suitably dissolving compounds containing relevant elemental components as a starting material in required amounts as in an aqueous medium, for example, heating and stirring them in the medium, then evaporating the resultant liquid mixture to dryness, and further pulverizing the resultant dry mixture when necessary.
Component (B):
The solid super acid as the component (B) is known, as remarked in the literature xe2x80x9cSHOKUBAIxe2x80x9d mentioned above, in two kinds, sulfuric acid-carried super acid and oxide-carried super acid. As typical examples of these kinds, the following super acids (1) through (7) may be cited.
(1) Super acids of SO4/oxide of a metal of Group IV in the Periodic Table of Elements:
In the metals of Group IV in the Periodic Table of Elements, zirconium, titanium, tin, and hafnium are favorably used. These metals may be used in the form of a mixture of two or more members. As typical examples of the super acid of this category, SO4/zirconium oxide, SO4/titanium oxide, SO4/tin oxide, and SO4/hafnium oxide may be cited. They are represented as SO4/ZrO2, SO4/TiO2, SO4/SnO2, and SO4/HfO2 respectively. These super acids are dealt with in such publications as xe2x80x9cAdvances in Catalysisxe2x80x9d, Vol. 37, pp. 182-191 (1990) and xe2x80x9cApplied Catalysisxe2x80x9d, Vol. 61, pp. 1 to 25 (1990) besides xe2x80x9cSHOKUBAIxe2x80x9d mentioned above.
The method for the preparation of these super acids will be explained below with respect to zirconium as an example. The SO4/zirconium oxide super acid is obtained by combining zirconium hydroxide or amorphous zirconium oxide with a sulfate ion-containing solution such as, for example, sulfuric acid or an aqueous sulfuric acid solution, then removing an excess of the sulfate ion-containing solution, subsequently drying the resultant wet mixture, and calcining the dry mixture in the atmosphere of an inert gas such as air or nitrogen gas at a temperature in the range of 350xc2x0 to 800xc2x0 C., preferably 400xc2x0 to 700xc2x0 C. for a period in the approximate range of 1 to 10 hours, preferably 2 to 8 hours. The super acid of other metal can be prepared by following this procedure while using the hydroxide or amorphous oxide of the metal instead.
It is generally held that, in the super acid which is obtained as described above, the sulfate ion (SO42xe2x88x92) is bound to or carried on a metal oxide. This super acid is expressed as SO4/metal oxide (MeOx) in the publications xe2x80x9cSHOKUBAIxe2x80x9d and xe2x80x9cAdvances in Catalysisxe2x80x9d mentioned above. The super acids which are used in the present invention, therefore, will be expressed after this manner.
(2) SO4/iron oxide super acid:
This super acid is expressed as SO4/Fe2O3 and is dealt with in the publications of xe2x80x9cSHOKUBAIxe2x80x9d and xe2x80x9cAdvances in Catalysisxe2x80x9d and in xe2x80x9cChemistry Lettersxe2x80x9d, pp. 1259 to 1260 (1979) as well.
This super acid is obtained by combining the hydroxide or amorphous oxide of iron with a sulfate ion-containing solution such as, for example, sulfuric acid or an aqueous sulfuric acid solution, then removing an excess of the sulfate ion-containing solution from the resultant liquid mixture, then drying the wet mixture, and calcining the dry mixture in the atmosphere of an inert gas such as air or nitrogen gas at a temperature in the range of 350xc2x0 to 800xc2x0 C., preferably 400xc2x0 to 650xc2x0 C. for a period in the approximate range of 1 to 10 hours, preferably 2 to 8 hours.
(3) SO4/silicon oxide super acid:
This super acid is expressed as SO4/SiO2 and dealt with in the publications of xe2x80x9cSHOKUBAIxe2x80x9d and xe2x80x9cAdvances in Catalysisxe2x80x9d mentioned above.
This super acid is obtained by combining silica gel with a sulfur-containing compound such as, for example, sulfuryl chloride, then drying the resultant liquid mixture, and subsequently calcining the dry mixture in the atmosphere of an inert gas such as air or nitrogen gas at a temperature in the range of 300xc2x0 to 600xc2x0 C., preferably 350xc2x0 to 500xc2x0 C. for a period in the approximate range of 1 to 10 hours, preferably 2 to 8 hours.
(4) SO4/aluminum oxide super acid:
This super acid is expressed as SO4/Al2O3 and dealt with in the publications of xe2x80x9cSHOKUBAIxe2x80x9d and xe2x80x9cAdvances in Catalysisxe2x80x9d mentioned above.
This super acid is obtained by combining y-alumina or aluminum hydroxide with a sulfate ion-containing solution such as, for example, sulfuric acid or an aqueous sulfuric acid solution, removing an excess of the sulfate ion-containing solution, then drying the resultant wet mixture, and subsequently calcining the dry mixture in the atmosphere of an inert gas such as air or nitrogen gas at a temperature in the range of 350xc2x0 to 800xc2x0 C., preferably 400xc2x0 to 700xc2x0 C. for a period in the approximate range of 1 to 10 hours, preferably 2 to 8 hours.
(5) Tungsten oxide, molybdenum oxide, or tungsten-molybdenum composite oxide/zirconium oxide super acid:
These super acids are expressed as WO3/ZrO2, MoO3/ZrO2, and WO3xe2x88x92MoO3/ZrO2 and dealt with in the publications of xe2x80x9cSHOKUBAIxe2x80x9d, xe2x80x9cChemistry Lettersxe2x80x9d, and xe2x80x9cAdvances in Catalysisxe2x80x9d and in xe2x80x9cJ. Chem. Soc., Chem. Commun.xe2x80x9d, pp. 1059 to 1060 (1988) as well.
These super acids are obtained by depositing a compound of tungsten and/or molybdenum on zirconium hydroxide or amorphous zirconium oxide and then calcining the resultant composite in the atmosphere of an inert gas such as air or nitrogen gas at a temperature in the range of 500 to 1,000xc2x0 C., preferably 650xc2x0 to 850xc2x0 C., for a period in the approximate range of 1 to 10 hours, preferably 2 to 8 hours.
The amount of tungsten oxide, molybdenum oxide, or tungsten-molybdenum composite oxide to be deposited is generally in the range of 1 to 40% by weight, based on the amount of zirconium oxide.
(6) Super acid of tungsten oxide/tin oxide, titanium oxide, iron oxide, or composite oxide of at least two elements selected among tin, titanium, and iron:
These super acids are expressed as WO3/SnO2, WO3/TiO2, WO3/Fe2O3, WO3/SnO2xe2x80x94TiO2, WO3/SnO2xe2x80x94Fe2O3, WO3/TiO2xe2x80x94Fe2O3, and WO3/SnO2xe2x80x94TiO2xe2x80x94Fe2O3 and dealt with in the publication of xe2x80x9cSHOKUBAIxe2x80x9d and in xe2x80x9cStud. Surf. Soc. Catal.xe2x80x9d, Vol. 75, pp. 2613 to 2616 (1953) as well.
These super acids are obtained by depositing a tungsten compound on at least one compound selected from the group consisting of stannic hydroxide, amorphous stannic oxide, titanium hydroxide, amorphous titanium oxide, ferric hydroxide, and amorphous ferric oxide and then calcining the resultant composite in the atmosphere of an inert gas such as air or nitrogen gas at a temperature in the range of 650xc2x0 to 1,200xc2x0 C., preferably 650xc2x0 to 1,000xc2x0 C., for a period in the approximate range of 1 to 10 hours, preferably 2 to 8 hours.
The amount of tungsten oxide to be carried is generally in the range of 1 to 40% by weight, based on the amount of the oxide such as tin oxide or titanium oxide.
(7) Super acid of phosphorus tungstate and/or an alkali metal salt thereof:
These super acids are expressed as H3P1W12O40 and H3 xe2x88x92xAxP1W12O40, [wherein A is an alkali metal (sodium, potassium, rubidium, and/or cesium) and x is above 0 and below 3 (0 less than x less than 3)]. These super acids are dealt with in xe2x80x9cChem. Tech.xe2x80x9d, November (1993), pp. 28 to 29,
These super acids are obtained by calcining phosphorus tungstate or alkali salts thereof in the atmosphere of an inert gas such as air or nitrogen gas at a temperature in the range of 350xc2x0 to 500xc2x0 C., preferably 380xc2x0 to 450xc2x0 C., for a period in the approximate range of 1 to 10 hours, preferably 2 to 8 hours.
As the component (B) according to this invention, two or more of the various super acids mentioned above may be used in a combined form.
Some of the solid acids to be used as the component (B) show degrees of acid strength of not more than xe2x88x9216.04 (Hoxe2x89xa6xe2x88x9216.04). These degrees cannot be definitely determined, however, because no methods have yet been established for determining acid strength less than xe2x88x9216.04, The super acids (1) through (7) mentioned above invariably show degrees of acid strength of not more than xe2x88x9211.93 and, therefore, can be effectively used as the component (B) of this invention.
The acid strength in this invention has been determined by the following method which is now in general use.
When a color of a sample subjected to the determination is white, this sample is immersed in benzene and a benzene solution containing an acid-base indicator of a known pKa value is added thereto. The sample is kept under observation until the indicator on the surface of the sample assumes the color of acidity. The smallest value of pKa at which the color of acidity is assumed is reported as the acid strength of the sample.
The indicators (pKa) which are usable for this determination include m-nitrotoluene (xe2x88x9212.0), p-nitrotoluene (xe2x88x9212.4), p-nitrochlorobenzene (xe2x88x9212.7), m-nitrochlorobenzene (xe2x88x9213.2), 2,4-dinitrotoluene (xe2x88x9213.8), and 1,3,5-trinitrobenzene (xe2x88x9216.0), for example.
When a given sample has a color, the sample is first placed in a container provided with a gas inlet and a gas outlet line. Then the container holding the sample is evacuated until thorough expulsion of the entrapped air and ammonia gas is introduced into the container and adsorbed on the sample. Then, the ammonia gas is gradually discharged from the container and meanwhile the temperature of the container is gradually elevated. The ammonia gas discharged at a varying level of the temperature in the container is collected with liquefied nitrogen. The amount of ammonia thus collected is determined on the weight of the sample. By rating this amount with the aid of a calibration curve separately obtained with samples of known degrees of acid strength, the acid strength of the sample is calculated.
Catalyst:
The catalyst of this invention contains the component (A) and the component (B) mentioned above. The amount of the component (B) based on the weight of the component (A) (as oxide) is generally in the range of 0.1 to 30% by weight, preferably 0.2 to 20% by weight. If the amount of the component (B) is less than 0.1% by weight, the effect of the addition of this component (B) will not be satisfactorily obtained. Conversely, if the amount exceeds 30% by weight, the activity of the catalyst will be found to decrease, the selectivity to acrylic acid from acrolein will be unduly small, and the selectivity to C2 and CO will be unduly large. To be specific, when a component (B) is used singly, a reaction of forming C2 and CO is easily carried out because the conversion of acrolein and the selectivity to acrylic acid are unduly low. The component (B), therefore, is a component to be disadvantageously used singly in the vapor-phase catalytic oxidation reaction according to this invention.
It, however, was demonstrated that by introducing a component (B) into a component (A), the selectivity to acrylic acid from acrolein and the activity by the use of the component (A) are improved. Particularly when a component (B) is introduced into a component (A) in such a proportion as mentioned above, the component (B) can prominently manifest its effects as a cocatalyst.
The catalyst which contains the component (A) and the component (B) is not particularly discriminated on account of the method adopted for its preparation. It can be prepared by any desired method. For example, the method which comprises first preparing the components each in a powdery form and then mixing them homogeneously by the use of a ball mill and the method which comprises causing the component (B) prepared in advance to be dispersed in the component (A) at any desired stage during the preparation of the component (A) may be adopted.
The catalyst is not particularly discriminated on account of its shape. It may be in any desired shape such as, for example, pellets, beads, cylinders, rings, and tablets. The average diameter of the catalyst particles is generally in the range of 1 to 15 mm, preferably 3 to 10 mm. In this case, in the deposited catalyst may be incorporated therein such inorganic fibers as glass fibers or various whiskers which are widely known to permit effective improvement of the strength and the attrition loss of the catalyst. Further, for the purpose of controlling the physical properties of the catalyst with high repeatability, such additives as ammonium nitrate, cellulose, starch, polyvinyl alcohol, and stearic acid which are generally known as powder binders may be used.
The catalyst of this invention can be used all by itself. Optionally, it may be used as deposited on an inert carrier such as, for example, alumina, silica-alumina, silicon carbide, titanium oxide, magnesium oxide, or aluminum sponge.
The catalyst of this invention is calcined at a temperature in the range of 300xc2x0 to 600xc2x0 C., preferably 350xc2x0 to 500xc2x0 C., for a period in the approximate range of 1 to 10 hours, preferably 2 to 8 hours.
Vapor-phase catalytic oxidation:
The vapor-phase catalytic oxidation reaction of acrolein or acrolein-containing gas according to this invention is not particularly discriminated and can be carried out by such methods as are generally adopted for this reaction mentioned above. For example, a mixed gas comprising 1 to 15% by volume, preferably 4 to 12% by volume of acrolein, 0.5 to 25% by volume, preferably 2 to 20% by volume of oxygen, 0 to 30% by volume, preferably 3 to 25% by volume of steam and 20 to 80 % by volume, preferably 50 to 70% by volume of an inert gas such as nitrogen gas is brought into contact with the catalyst of this invention at a temperature in the range of 180xc2x0 to 350xc2x0 C., preferably 200xc2x0 to 330xc2x0 C., under a pressure in the range of normal pressure to 10 atmospheres (or naturally under a reduced pressure), preferably normal pressure to 8 atmospheres, at a space velocity (STP) in the range of 500to 20,000 hrxe2x88x921, preferably 1,000 to 10,000 hrxe2x88x921.
As raw material gases which are usable in this invention, in addition to a mixed gas comprising acrolein, oxygen and an inert gas, an acrolein-containing mixed gas obtained by directly oxidizing propylene and optionally combining with air or oxygen, and steam in advance can be used. Such oxidized products as acrylic acid, acetaldehyde and acetic acid, carbon oxide, propane and unaltered propylene which are by-products contained in the acrolein-containing mixed gas obtained by directly oxidizing propylene have no adverse influences on the catalyst composition to be used in this invention.
The method of this invention can be carried out either on a solidified bed or a fluidized bed.
Function:
The physical and chemical differences between a catalyst with a lowered activity and a virgin catalyst such as of various changes of physical properties of the composite oxide catalyst represented by the general formula (1) as a component (A), e.g., a change of surface area or pore volume, and a change of acidity shows a correlation of the change with time of properties of the catalyst with the changes of acid amount and base amount to be determined from the amount of propylene and acetone produced via decomposition reaction of isopropyl alcohol. The acid amount determined through the medium of the decomposition reaction of isopropyl alcohol is equal to the amount of propylene produced. The base amount similarly determined is equal to the ratio of the amount of produced acetone/the amount of produced propylene. When the surface areas and pore volumes of the virgin catalyst of Mo12V5.5W1Cu2.5Sr0.25 (atomic ratios except for that of oxygen) and the catalyst having the properties thereof deteriorated by the continuous catalytic vapor-phase oxidation reaction of acrolein for a period of 8,000 hours were tested, the surface areas determined by BET method of the virgin catalyst and the catalyst after the reaction for 8,000 hours were respectively 3 m2/g and 2.8 m2/g which are not outstandingly different. Similarly, the pore volumes of the virgin catalyst and the catalyst after the reaction for 8,000 hours were respectively 0.23 cc/g and 0.22 cc/g which are not outstandingly different neither. On the other hand, when the acid amount and base amount to be produced by the decomposition reaction of isopropyl alcohol were measured, the acid amount (amount of propylene produced) and base amount (amount of produced acetone/amount of produced propylene) of the catalyst after the reaction for 8,000 hours were respectively 1.0 mmol/g of catalyst and 0.8 per hour as compared with those of the virgin catalyst of 2.5 mmols/g of catalyst and 0.5, Other words, the acid amount is extremely decreased and the base amount is increased after the use of the catalyst. This result shows the relationship of the changes of the acid amount and base amount as one cause of the decrease of activity of the catalyst. The service life or the stability of properties of the catalyst can be estimated by determining the acid amount and base amount produced by decomposition reaction of isopropyl alcohol, and comparing these amounts before and after the reaction (see Example 20 and Control 3).
The function of the component (B) in the catalyst of this invention has not yet been fully elucidated. It is, however, inferred that the component (B) contributes to the high yield because the strong acidity of the component (B) promotes the adsorption of such reactive substances as acrolein on the catalyst and consequently exalts the activity of the catalyst and also promotes the elimination of acrylic acid as a product, and further because the component (B) represses the production of such by-products as acetic acid and maleic acid. It is further considered that the component (B) contributes to stabilize the composite oxide of the component (A) because the component (B) possesses a large surface area and moreover excels in resistance to heat. It should be noted, however, that this invention is not restricted by such theoretical consideration as mentioned above.
Now, this invention will be more specifically described below with reference to working examples. The terms xe2x80x9cconversion of acroleinxe2x80x9d, xe2x80x9cselectivity to acrylic acidxe2x80x9d, and xe2x80x9cper pass yield of acrylic acidxe2x80x9d are defined as follows.
Conversion of acrolein (%)=[(Number of mols of acrolein reacted)/(Number of mols of acrolein fed to the reaction)] (xc3x97100)
Selectivity to acrylic acid (%)=[(Number of mols of acrylic acid formed)/(Number of mols of acrolein reacted)] (xc3x97100)
Per pass yield of acrylic acid (%)=[(Number of mols of acrylic acid formed)/(Number of mols of acrolein fed to the reaction)] (xc3x97100)
Further, the acid strength of the solid acid was measured in the same manner as mentioned above.